The invention relates to the reception of a radionavigation signal originating from a satellite-based positioning system such as the GPS system (acronym of the expression “Global Positioning System”).
To operate properly, contemporary receivers generally require direct line of sight satellite reception. Positioning rapidly deteriorates especially in terms of accuracy and acquisition time, when reception is disturbed as is the case inside a building or more generally in a degraded environment.
The operation of the GPS system will now be recalled briefly. It consists of a constellation of 28 satellites and of a terrestrial network of reference land stations. Each satellite orbits the earth at 20,000 km with a period of revolution of 12 h. They send two signals, one at 1575.452 MHz for civil applications and the other at 1227.6 MHz for reserved access applications. Hereinafter, only the civil frequency will be considered. The signal sent by a satellite consists of a carrier of frequency 1575.452 MHz, modulated by a known spreading code and possibly by unknown data also called data bits. The satellites all send on the same frequency and the signals sent are differentiated through their code.
These codes have a period T, for example 1 ms and typically consist of 1023 chips.
The positioning of the receiver is obtained by measuring the distance between a satellite and the receiver on the basis of the signal propagation time between this satellite and the receiver. In the receiver, a replica of the code sent is generated locally; the shift between the signal received and the local signal (that is to say the replica) corresponds to the sought-after propagation time. This shift is measured by placing the signal received and the local signal in phase; the criterion of placing in phase corresponds to the maximization of the correlation function of the two signals, that is to say to the search for a correlation peak.
This correlation calculation is generally performed from code half-chip to code half-chip over an integration interval that it is possible to vary. For an integration interval of 1 ms, a correlation calculation time of about 2 s is obtained. (2×1023×1 ms=2 s). This calculation is multiplied by a factor K dependent on the drift of the local clock (or pilot) of the receiver and the number of assumptions about the frequency of the signal to be considered in order to compensate for the Doppler effect. For a clock uncertainty of about ±10 kHz, K=21; a calculation time of about 2 s×21 is then obtained, i.e. 42 s per code, that is to say per satellite.